1. Field of the Invention
This invention relates generally to oil pressure control systems for power transmissions, and more specifically to an oil pressure control system for a power transmission in a heavy duty vehicle, for example, a forklift which modulates an oil pressure supplied to a clutch servo controlling the power transmission.
2. Description of the Prior Art
A conventional oil pressure control system for a power transmission is disclosed in the Japanese Laid-Open patent application No. 59-81231 (1984), wherein as shown in FIG. 1, an oil pressure control system includes an oil pump 101 as a pressure source, a regulator valve 102, an inching valve 103, a selector valve 104, a change over valve 105, a modulator valve 106, a plurality of clutch servos 107 and 108, a first oil passage 109 for communicating the oil pump 101 with the selector valve 104 via the regulator valve 102 and the inching valve 103, a second oil passage 110 for communicating the selector valve 104 with the change over valve 105 and clutch servos 107 and 108, a third passage 111 for communicating the change over valve 105 with one end of the modulator valve 106, and a fourth oil passage 112 for communicating the change over valve 105 with the other end of the modulator valve 106. An oil pressure introduced to the second oil passage 110 from the oil pump 101 via the first oil passage 109 is introduced to the modulator valve 106 via the third and fourth passages 111, 112 and modulated by the modulator valve 106. The modulated oil pressure in the second oil passage 110 is supplied to the clutch servos 107 and 108. In such conventional systems, a pressure loss is caused at a branch point between the clutch servos 107 and 108 and the side of change over valve 105 within second oil passage 110 and an inlet portion of modulator valve 106 within the third oil passage 111. Therefore, in order to compensate for the pressure loss, oil pressure at the modulating valve 106 is controlled so as to obtain a proper operating oil pressure for supplying to the clutch servos 107, 108.
However, the pressure loss is not a constant because the oil pressure condition changes with respect to the selected position of selector valve 104, and the oil pressure is affected by the change over valve 105. Consequently in conventional fluid pressure control system, the operation of clutch servos 107 and 108 is not stabilized.